Chemical-mechanical planarization ("CMP") processes remove material from the surface of a wafer in the production of ultra-high density integrated circuits. In a typical CMP process, a wafer is pressed against a polishing pad in the presence of a slurry under controlled chemical, pressure, velocity, and temperature conditions. The slurry solution generally contains small, abrasive particles that abrade the surface of the wafer, and chemicals that etch and/or oxidize the surface of the wafer. The polishing pad is generally a planar pad made from a relatively soft, porous material such as blown polyurethane. Thus, when the pad and/or the wafer moves with respect to the other, material is removed from the surface of the wafer by the abrasive particles (mechanical removal) and by the chemicals (chemical removal) in the slurry.
FIG. 1 schematically illustrates a conventional CMP machine 10 with a platen 20, a wafer carrier 30, a polishing pad 40, and a slurry 44 on the polishing pad. The platen 20 has a surface 22 upon which the polishing pad 40 is positioned. A drive assembly 26 rotates the platen 20 as indicated by arrow "A" and/or reciprocates the platen 20 back and forth as indicated by arrow "B". The motion of the platen 20 is imparted to the pad because the polishing pad 40 is attached to the surface 22 of the platen 20 with an adhesive. The wafer carrier 30 has a lower surface 32 to which a wafer 60 may be attached, or the wafer 60 may be attached to a resilient pad 34 positioned between the wafer 60 and the lower surface 32. The wafer carrier 30 may be a weighted, free-floating wafer carrier, or an actuator assembly 36 may be attached to the wafer carrier 30 to impart axial and rotational motion, as indicated by arrows "C" and "D", respectively.
In operation of the conventional planarizer 10, the wafer 60 is positioned face-down against the polishing pad 40, and then the platen 20 and the wafer carrier 30 move relative to one another. As the face of the wafer 60 moves across the planarizing surface 42 of the polishing pad 40, the polishing pad 40 and the slurry 44 remove material from the wafer 60.
CMP processes must consistently and accurately produce a uniform, planar surface on the wafer to enable precise circuit and device patterns to be formed with photolithography techniques. As the density of integrated circuits increases, it is often necessary to accurately focus the critical dimensions of the photo-pattern to within a tolerance of approximately 0.1 .mu.m. Focusing the photo-patterns to such small tolerances, however, is very difficult when the distance between the photolithography energy source and the surface of the wafer varies due to non-uniformities on the wafer. Thus, CMP processes must create a highly uniform, planar surface.
The surface of a wafer, however, may not be uniformly planar because the rate at which the thickness of the wafer decreases as it is being planarized (the "polishing rate") often varies from one area of the wafer to another. The polishing rate is a function of several factors, some of which are: (1) the uniformity of the slurry distribution across the surface of the wafer; (2) the surface contact rate between the polishing pad and the wafer; and (3) the extent to which residual materials aggregate near the center of the wafer. The slurry distribution varies across the face of the wafer because the perimeter of the wafer scrapes the slurry off the planarizing surface. Therefore, only a thin layer of slurry remains on the pad at the center of the wafer. The surface contact rate also varies across the face of the wafer because the linear velocity of the pad varies from the center of the pad to its perimeter. Lastly, residual particles of planarized wafer material and pieces of the pad can, for example, aggregate at the center of the wafer and form a barrier between the surface of the wafer and the slurry. The barrier of residual materials accordingly reduces the polishing rate at the center of the wafer. Therefore, in light of the above-listed problems, it would be desirable to enhance the slurry distribution, equalize the contact rate, and reduce the amount of residual materials on the surface of the wafer.
U.S. Pat. Nos. 5,020,283 to Tuttle, 5,293,364 to Tuttle, and 5,232,875 to Tuttle et al. disclose several existing polishing pads that enhance the slurry distribution and equalize the contact rate across the face of the wafer. The above-listed patents disclose polishing pads that have a face shaped by a series of voids to provide a nearly constant surface contact rate between the pad and the wafer. The voids also enhance the slurry distribution across the face of the wafer because they hold a small volume of slurry that is not scraped off the pad by the perimeter of the wafer. The above-listed patents, however, do not significantly reduce the amount of residual materials on the wafer.
Another objective of CMP processes is to minimize the number of defects on the finished planarized surface. The surface of the wafer is often damaged during the planarization process because residual particles from the pad or the wafer scratch the surface of the wafer. Thus, it would be desirable to develop a pad that reduces surface damage caused by residual particles.